## Cryptology ePrint Archive: Report 2004/316

On a Threshold Group Signature Scheme and a Fair Blind Signature Scheme

Zhengjun Cao

Abstract: In the paper, we analyze two signature schemes. The first is a $(t_j, t, n)$ threshold group signature scheme proposed by Shi and Feng in [1]. The second is a fair blind signature scheme proposed by Feng in [2]. Our results show that both schemes are forgeable. Besides, we introduce a concept, i.e., suspended factor, to describe the common error in designing signature scheme, which means that some signature data lie at neither base position nor exponent position in verifying equation, instead lie at factor position solely .

Category / Keywords: cryptographic protocols / threshold group signature scheme, fair blind signature scheme, universal forgeability, suspended factor.